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An ageing population will also have an effect. To date, around eight out of ten workers registering with the Home Office have been aged 18-34.

In Poland, however, the population of 18-year-olds will fall from 600,000 in 2005 to 400,000 in 2016.

Also, countries which blocked access to Eastern Europeans in 2004 - unlike Britain - will have to open their borders by 2011 at the latest.

This means migrants who may otherwise have headed to Britain will move to France, Germany or Austria instead.

Migrationwatch's report follows statistics published by the Government last week showing the number of Eastern Europeans coming to work in the UK had fallen to its lowest level since EU expansion.

Return ticket?: Migrants from the Eastern Bloc are starting to head home

But chairman Sir Andrew Green said that Eastern Europeans have never accounted for more than a third of the immigrant population.

He said the Government now has an opportunity to bring migration under control, if it had the ' political will'.

Ministers have no control over immigration within the EU, but they are free to limit the number of work permits given to others.

'The latest population estimates confirm that, in the next 25 years, we will have to build seven cities the size of Birmingham just for new immigrants. This cannot be allowed to continue unchecked,' said Sir Andrew.

'The Government is carrying out a major reform of the immigration system but its central feature, the points-based system, does not limit immigration and is not even intended to do so.'

A Home Office spokesman said: 'The tough new Australian-style points-based system means only those we want and no more can come here to work and study.

'The points system covers close to six in ten of all migrants - more than a cap - and is flexible, allowing us to raise or lower the bar according to the needs of business and the country as a whole.'

Addressing fears that a drop in immigrant numbers could hit the UK's workforce, she added: 'Nationals from the Accession 8 countries (those that joined the EU in 2004) continue to come to the UK to work, contributing to the success of the UK economy, whilst making few demands of our welfare system.

'Migrant labour is by no means the only solution to our tight labour market. The National Skills Strategy was launched to address the needs of our domestic labour force.'